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9 Things Everyone Should Know About Clarifiers

Yes, everyone. What? You thought learning about water treatment was only for engineers, scientists and magnificent captains of industry? You’re probably right, but we here at Durpro (I didn’t ask the others) really like learning new stuff so you thought you might like this post even if it’s not directly related to your job.

Where I come from (Montreal), there was recently a pretty big stink about the city dumping a bunch of untreated waste water back into the St. Lawrence river. Of course, everyone has forgotten about that whole fiasco by now. So what the heck does Montreal dumping a bunch of untreated sewage into our beloved St. Lawrence river have to do with learning about clarifiers? Well, cities rely quite a bit on different water clarification methods (mostly settling) to clean up both the water we drink AND the water we send back to the environment.

Here are some things you should know if you want to get lots of contaminants out of lots of water.

There are many types of clarifiers

In order not to sound like Bubba from Forest Gump I won’t go on listing different types of clarifiers forever, but you should be aware of at least 3 types:

  1. Clarifiers that let junk sink to the bottom (settling clarifiers)
  2. Clarifiers that let (or help) stuff rise to the top (dissolved air flotation clarifiers, among others)
  3. Clarifiers that use centrifugal force to fling stuff out of the clean water stream (centrifugal clarifiers)

Specific gravity matters in clarification

Ok, for you non-science types or people like me who got an engineering degree in a weird abstract field like software engineering, take 5 minutes to read about specific gravity on Wikipedia. Now that you have a headache, specific gravity is really just how “heavy” something is in comparison to another. In clarification we usually want to know if a contaminant is heavier or lighter than water to know whether it will tend to sink, float to the surface or just hang out.

So it’s pretty clear why settling and floating matter, but why does specific gravity matter in centrifugal clarifiers? Simple: centrifugal force works just like gravity. So if you have a bunch of contaminants with different specific gravities in your waste stream, the ones with the highest specific gravity (heaviest) will be flung outwards faster than the lower specific gravity contaminants. Rocks will fling outward faster than chicken feathers (we know, we’ve put both through clarifiers).

Clarification works for dissolved solids too

So you’ve heard of dissolved solids and suspended solids right? We won’t go into detail here but just know that suspended solids are basically solids you can spot with your naked eye or under a normal microscope while dissolved solids are broken down to the molecular level within a liquid. Chemists and chemical engineers, feel free to flog me for oversimplification in the comments below.

Anyway, if you want to get dissolved solids out as well as suspended solids you’ve got a few options like RO, ion exchange, and so on. In cases where you want to get lots of stuff out of lots of water, though, your best bet is to use clarification. Those dissolved solids won’t just come out on their own though. They need help. This is where coagulation and flocculation come in. Coagulants help get dissolved solids out of solution and flocculants help those tiny, newly suspended particles to stick together and form bigger easier to manipulate clusters.

Clarifiers are time sensitive

Clarification takes time. All those particles, feathers, scrambled eggs (just ask, I’ll send a video) and other junk need time to float or sink. That’s why most settling or floating clarifiers need to be so large. If a pulp and paper mill is generating 900 gallons per minute of waste water and they need to let things settle for 10 minutes that means their clarifier will need a volume of at least 9000 gallons (1203 cubic feet).

Centrifugal clarifiers are a bit different and can usually have a smaller footprint. That said, the water entering a centrifugal clarifier must be traveling in a very tight velocity range so flows greatly impact sizing and other design considerations.

Floating works best with tiny bubbles

Just like tiny bubbles in the wine make me happy… make me feel fine… they do the same for owners of dissolved air clarifiers. If you ever shop for a dissolved air clarifier (clarifiers that help stuff float by generating bubbles that cling to the contaminants) you’ll want to make sure that the clarifier you buy generates the smallest bubbles possible. Small bubbles cling better to suspended solids and also provide more buoyancy (floating power). Big bubbles just rush their way up to the surface, creating agitation and missing everything on their way up.

Our Dur-flote DAFs generate micro bubbles by actually dissolving air into water under pressure and then releasing the pressure. Think about shaking a bottle of Pepsi just before opening it. That’s the kind of effect we’re going for.

Clarifiers are sensitive to flow upsets

The city of Montreal couldn’t contain all that raw sewage because it needed to release it in too short a period of time. Its clarifiers and other waste treatment installations just couldn’t keep up to a significant flow increase.

In the case of centrifugal and dissolved air flotation clarifiers, flow is critical. If flows are too low, centrifugal clarifiers don’t have enough “oomph” to fling contaminants outwards and DAFs will have their bubbles burst (literally) and let contaminants sink into the “clean” water stream. If flows are too high, DAFs will become turbulent and contaminants won’t have enough time to float while centrifugal clarifiers will see other difficult to handle upsets.

Clarification is not filtration

Filtration involves putting a physical barrier in place that will stop particles of a given size from passing through. Usually, filtration is rated with a specific efficiency at a certain micron rating. For example, a specific filter could be rated to stop 99.95% of particles 1 micron or larger in size. Clarifiers offer no such physical barrier and generally can’t be guaranteed to provide a specific removal efficiency because changing operating conditions and contaminant loads will greatly influence their removal capacity.

Clarification can seriously impact downstream filtration and purification

Really, this is true of most technologies that aim to remove contaminants from water, but we include it here because clarifiers are particularly sensitive to upsets, timing and effective application of water treatment chemistry.

Using a clarifier ahead of an RO system, for example, could result in blinding (plugging) of the RO membranes if certain coagulants or flocculants make it through to the membranes before they have time to react with contaminants and cling to larger particles.

An upset that causes bad effluent quality could plug downstream filters in a heartbeat. Clarifiers must be extremely well understood and well controlled when they’re being used to feed filtration systems or critical equipment like paper machine showers.

Clarification is extremely efficient when mastered

We don’t want you to think that clarification is some kind of impossible to use technology. Humans have been using clarifiers for hundreds (thousands?) of years with great success. If you’re having trouble with a clarifier or if you’re thinking of installing a new clarifier, just make sure you’ve got a partner that comes well recommended to help you in selection, design, installation and operation.

Clarifiers are among the most efficient technologies for removing large quantities of contaminants from large volumes of water. If you’re in one of the top water-consuming industries, your plant probably already owns and operates at least one clarifier.

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